1. Field of the Invention
The present invention relates to oxygen permeable hard contact lenses, and more specifically to oxygen permeable hard contact lenses formed of copolymers comprising specific compounds as constituents.
2. Description of the Prior Art
Heretofore, soft contact lenses formed of polymers comprising 2-hydroxyethyl methacrylate or vinylpyrrolidone as their principal constituent, which contain water, and hard contact lenses formed of polymers comprising as their principal constituent methyl methacrylate (MMA), fluoroalkyl methacylate or silicone methacrylate, which do not contain water, have been available on the market.
However, conventional water-containing soft contact lenses mentioned above have low oxygen permeabilities, and hence they cannot be worn over a long period of time or are unusable due to their low mechanical strength.
On the other hand, conventional hard contact lenses, which do not contain water, comprising MMA as their principal constituent have almost no oxygen permeabilities, and hence they have a significant drawback of being difficult to continuously wear over a long period of time. Conventional hard contact lenses comprising silicone methacrylate as their principal constituent are, while they have higher oxygen permeabilities than those comprising MMA as principal constituent, still insufficient for a long-period continuous use and low in wettability and further have a drawback of being readily soiled with lipids and the like.
In recent years, several contact lenses have been proposed to solve the above-mentioned problems, such as one containing an increased amount of silicone methacrylate and one containing fluorine atom. For example, Japanese Patent Publication No. 8769/1987 and Japanese Patent Application Laid-open No. 28127/1984 disclose an oxygen permeable hard contact lense comprising fluoroalkyl methacrylate and siloxanylalkyl (glycerol) (meth)acrylate. Japanese Patent Application Laid-open Nos. 131518/1985, 57612/1986, 281116/1986, 281216/1986, 285426/1986 and 54220/1987 disclose an oxygen permeable hard contact lens comprising siloxanylpropyloxy ethyl (meth)acrylate and alkyl (meth)acrylate or alkyl itaconate. Further Japanese Patent Application Laid-open Nos. 87102/1986 and 38419/1987 disclose an oxygen permeable hard contact lens comprising fluoroalkylsiloxanyl (glycerol) (meth)acrylate.
U.S. Pat. No. 4,711,943 discloses a water-containing soft contact lens formed of a material having both an amide structure and a siloxane structure and having water content of 15 to 60%, an oxygen permeability of at least 25.times.10.sup.10 cm.sup.3.cm/cm.sup.2.sec.cmHg, a tear strength of at least 1.0 g/mm.sup.2 and an elongation of at least 80%.
In any one of the above improvements, i.e. the increased content of silicone methacrylate, the incorporation of fluorine atom or incorporation of fluorine atom together with silicone methacrylate, the acquired increase in oxygen permeability always leads to reduction in wettability, thereby rendering the lens cloudy or readily to be soiled by lipids and the like during use. Accordingly, there has been desired an oxygen permeable hard contact lens which maintains both high oxygen permeability and wettability and at the same time has high mechanical strength, transparency and resistance to soiling.